A semiconductor device having an IGBT has been known as a power converter used in an electronic apparatus such as an industrial motor. A typical structure of such a semiconductor device is as follow.
An N−-type drift layer is formed on a P+-type semiconductor substrate as a collector layer. A P-type base layer is formed in a surface portion of the N−-type drift layer. An N+-type emitter layer is formed in a surface portion of the P-type base layer. Trenches penetrating the P-type base layer and the N+-type emitter layer and reaching the N−-type drift layer are arranged in a stripe pattern. A gate insulation layer and a gate electrode are formed on a wall of each trench so that a trench gate structure can be formed. An emitter electrode is formed on the P-type base layer and the N+-type emitter layer through an interlayer dielectric layer. The emitter electrode is electrically connected to the P-type base layer and the N+-type emitter layer through a contact hole formed in the interlayer dielectric layer. A collector electrode is formed on a back surface of the collector layer and electrically connected to the collector layer.
In such a semiconductor device, when a predetermined gate voltage is applied to a gate electrode, an inversion layer is formed in a portion of the P-type base layer in contact with the gate insulation layer, and an electron accumulation layer is formed in a portion of the N−-type drift layer in contact with the gate insulation layer. Then, electrons flow from the N+-type emitter layer to the N−-type drift layer through the inversion layer and the accumulation layer, and holes flow from the collector layer to the N−-type drift layer. Thus, a resistance decreases due to conductivity modulation so that the semiconductor device can be turned ON.
Although an ON-voltage of the semiconductor device having such an IGBT is smaller than that of a semiconductor device having a metal-oxide semiconductor field-effect transistor (MOSFET), there has been an increasing demand to further reduce the ON-voltage.
In the semiconductor device disclosed in US 2007/0001263 corresponding to JP-A-2007-43123, the distance between adjacent gate electrodes is set to a very small value ranging from 0.55 nm to 0.3 μm.
In the semiconductor device disclosed in JP-A-2008-153389, the width of the bottom of the trench gate structure is greater than the width of the other portion of the trench gate structure so that the distance between the bottoms of adjacent trench gate structures can be smaller than the distance between the other portions of the trench gate structures.
In such a semiconductor device as disclosed in US 2007/0001263 or JP-A-2008-153389, it is less likely that holes flowing into the N−-type drift layer flow to the P-type base layer through a space between adjacent trench gate structures. Thus, a lot of holes can be accumulated in the N−-type drift layer. Thus, the amount of electrons injected from the emitter layer into the N−-type drift layer through the inversion layer and the accumulation layer is increased. Since the electron mobility is greater than the hole mobility, the ON-voltage can be further reduced.
By the way, there has been an increasing demand to improve a load short-circuit capability of a semiconductor device while reducing an ON-voltage of the semiconductor device.
That is, when a load is short-circuited, a large saturation current flows so that Joule heat proportional to the saturation current can be generated. As a result, a temperature of the semiconductor device may be increased above the maximum allowable temperature.